Danilo Makuc

Modeling, analysis and detection of rotor field winding faults in synchronous generators

The paper presents an approach to modeling of shorted turns in rotor field winding of synchronous generator using finite element method. It enables detailed analysis of magnetic field at several operating conditions under healthy and faulty states which are difficult or even impossible to carry out by available measurement methods in industrial environment. Modeling of field winding faults are performed for both typical generator designs - turbo and hydro, and analysis reveals some differences, which are significant for practical use in fault detection procedures. It is confirmed that an extensive analysis should be performed to assure accurate healthy/faulty state predictions, since the level of diagnostic signal is considerably influenced by a combination of many machine and operating parameters. The scheme of developed on-line diagnostic system with its hardware and software concept is also presented and discussed.

Simplified model of induction machine with electrical rotor asymmetry

In this paper, a simplified two-axis model for induction machines with electrical rotor faults is derived. The model is intended for analysis of the impact of rotor fault on control loop with inverter fed induction machines, therefore detailed modeling of internal states of the machine is not required. The model is derived from unified theory of electrical machines, where rotor parameters are asymmetrical. The assumptions made during the development of the model are confirmed by experimental measurements. Finally, the comparison between proposed simplified model and detailed induction machine model is performed. The comparison shows very good agreement between simplified and detailed induction machine model, thus making the proposed model suitable for simulations of rotor faults.

Computations of Magnetic Field Anomalies in Synchronous Generator Due to Rotor Excitation Coil Faults

The paper presents an approach to model shorted turns in rotor excitation winding of synchronous generator using finite element analysis. It enables detailed examination of magnetic field at several operating conditions under healthy and faulty states which are difficult or even impossible to carry out by available measurement methods in industrial environment. It is confirmed that an extensive analysis should be performed to assure accurate healthy/faulty state predictions, since the level of diagnostic signal is considerably influenced by a combination of many machine design and operating parameters.

Pseudo-salient model of induction machine with broken rotor bars

In this paper, a dynamic model of a squirrel cage induction motor with broken rotor bars, is presented. The model is based on the assumption that the asymmetry due to broken bars is reflected through different resistances and inductances in a two-phase rotor model. Their joint effect is best expressed through rotor time constant. Mathematical equations, based on this premise, have been developed in rotor reference frame, thus obtaining a very simple yet effective model of the machine. Additionally, a method for measurement of both time constants of a faulty rotor at standstill is explained. Simulation results, based on this model, have been experimentally verified for healthy and faulty machine, showing very good agreement at different operational points. Finally, a study of the impact of various relationships between the parameters in both rotor axes on the behavior of the simulation model, is performed.

Estimation of parameters of induction motor with broken rotor bars

This paper describes the evaluation of parameters of a three-phase induction motor with broken rotor bars. Such fault causes electric and magnetic asymmetry in the machine. Using standard d-q model of induction motor this asymmetry can be expressed through differently modified rotor parameters in both axes. In pursuing the goal to establish accurate motor parameters, a non-standard, one-phase measuring method was employed. In this way angle dependence of rotor parameters following a sinusoidal shape was determined. Additionally to measurements an extensive finite element analysis has been performed for two purposes. Firstly, to assess the measurement results of the method and, secondly, to predict various situations which can not be easily achieved by measurements due to hardware limitations (e.g. different numbers of broken bars). Finite element analysis evidences the correctness of the machine parameters obtained from proposed measurement method.

Induction motor parameters in case of rotor electrical asymmetry

The paper presents estimation of parameters alternation in a three-phase induction motor due to broken rotor bars, which cause electric and magnetic asymmetry in the machine. Using d-q model of induction motor this asymmetry is reflected as pseudo-saliency through different resistances and inductances in both axes. An extensive study has been performed to simulate various cases of faulty cage, which cannot be achieved by measurements on actual industrial drives. Simulation results can be efficiently used for analysis of closed-loop induction motor drives, in which determination of rotor faults, due to the intrinsic speed correction performed by the controller, is much harder to achieve.

Electric Field Analysis of the Insulation Structure of Power Transformer

Electric field and dielectric conditions were analysed in an oil immersed power transformer. The particularity of the transformer’s design we are dealing with is an attempt to eliminate the stress-control ring above the low voltage (LV) winding. Using the finite element method (FEM) we analysed the steady state electric field that suited conditions during dielectric test with a separate AC source. In the paper is also discussed the evaluation and analyses of other possibilities to form an adequate electric field with simple constructional elements at the ends of the windings. Therefore we evaluated the influence that the radius of conductors, thickness of paper insulation, and additional insulation elements have on electric field strength in the critical places.

Impact of production tolerances on losses in electrical machines

The aim of the paper is to show the importance of paying an attention to precisely manufacture electric machines. The study is done on axial flux permanent magnet prototype machine. The model has also been analyzed by using 3D finite element method. During generator mode no-load testing of the prototype machine, high unexpected overheating of the rotor discs was revealed. Further investigations showed that the cause of the heating were misalignments in the machine prototype production.

Calculation of Linear Motor Performance Using Finite Element Method

The paper presents calculation of linear induction motor and linear synchronous motor steady- state characteristics using finite element method. 2D non-linear magnetostatic and eddy-current solver were used for magnetic field and force calculation. A detailed insight in magnetic field distribution and produced output force is a basic step for evaluation of motor performance. Calculations were performed for different excitation currents and different air gap lengths for both types of linear motor. The results obtained from the magnetic field calculations could also be used for determination of motor parameters (e.g. inductance) useful for mathematical modelling and control system simulations.